SIGNIFICANCE. Aging is associated with increased body fatness and risk of cardiovascular disease. This gain in body fat with aging may be attributable to decrements in daily energy expenditure, increases in energy intake and/or a combination of these factors. To date, no longitudinal studies have examined the influence of total daily energy expenditure and it's components (i.e., resting metabolic rate; physical activity energy expenditure) on body composition changes in older women and men. Moreover, genetics may influence the heterogeneity of age-related changes in energy expenditure and body composition in aging women and men. A variant of the B3-adrenergic receptor gene has been recently discovered and implicated in the regulation of energy expenditure and fat metabolism. Cross-sectional studies have shown that individuals with this variant have a greater body mass index and a lower resting metabolic rate. Thus, it is possible that the rate of change in energy expenditure and body composition with aging may be different in individuals with the B3- adrenoceptor variant. To our knowledge, no longitudinal studies have examined the influence of the B3-adrenoceptor variant on energy expenditure and body composition changes in older women and men. AIMS. The specific aims of this project are: Aim #1: To examine the rate of change in total daily energy expenditure and its components (resting metabolic rate, energy expenditure of physical activity), and body composition in older women and men. Aim #2: To examine longitudinal changes in total daily energy expenditure and its components (resting metabolic rate, energy expenditure of physical activity), and body composition in older individuals who are homozygous/heterozygous for the B3-adrenoceptor variant. APPROACH: We have previously tested 434 women and men between 40 and 90 years of age. Each subject was retested on average 5 y ago (range = 4-8 y) and will be retested during an overnight, inpatient visit on the General Clinical Research Center at the University of Vermont. Subjects will be retested within 30 days of their previous testing date to match for seasonality. The same techniques that were previously used to test these subjects are currently being used in our laboratory. Each subject must pass a comprehensive medical examination before participating in the retesting. Total daily energy expenditure and physical activity energy expenditure will be measured over 10 days with doubly labeled water. Resting metabolic rate will be measured using indirect calorimetry. Body composition will be assessed with underwater weighing. Central adiposity will be estimated from waist circumference measurements. Aerobic fitness will be measured using an incremental treadmill protocol to volitional exhaustion. Energy intake will be assessed from a 3-day dietary record. Furthermore, all of the 434 subjects were previously tested for resting metabolic rate, body composition, and aerobic fitness. In contrast, only 90 subjects (47 women and 43 men) were previously measured for total daily energy expenditure and its components for comparison with values obtained during the retesting. PROGRESS (12/1/96 - 11/30/97): Data collection for this project began in June of 1997 and we have been able to retest 64 subjects. We have encountered no trouble during this period in re-recruiting subjects and completing all testing. FUTURE: We will continue to recruit subjects throughout 1998 with hope of retesting an additional 100 subjects. This protocol is funded through July of 2000. It is anticipated that we will be able to publish preliminary data from this study in the fall of 1998.